In a number of applications, it is desirable to interconnect two bundles of optical fibers, each of which includes a plurality of optical fibers. Moreover, it is frequently desirable that at least one of the bundles of fibers be permitted to rotate, such as about the longitudinal axis of the bundle, relative to the other bundle of optical fibers. As such, fiber optic rotary joints are employed to appropriately interconnect respective optical fibers of two different bundles, while permitting at least one of the bundles to rotate relative to the other. See, for example, U.S. Pat. No. 6,301,405 to Mitchel J. Keil, U.S. Pat. No. 5,442,721 to Gregory H. Ames and U.S. Pat. No. 5,271,076 to Gregory H. Ames, the contents of each of which are incorporated herein in their entirety.
A fiber optic rotary joint includes a housing that defines an internal cavity. The housing is adapted to engage end portions of two bundles of optical fibers, hereinafter referenced as the first and second bundles. As such, the first and second bundles of optical fibers are typically disposed on opposite sides of the internal cavity. By appropriately aligning the first and second bundles of optical fibers, respective pairs of the optical fibers of the first and second bundles can communicate across the internal cavity. In order to assist with this alignment and to facilitate communications between the first and second bundles of optical fibers, the fiber optic rotary joint includes a reversion prism. A reversion prism is a trapezoidal prism defining a longitudinal axis therethrough and having opposed end faces that are disposed at equal, but opposite, angles relative to the longitudinal axis. As such, optical signals emitted by an optical fiber of the first bundle are refracted by one angled end surface of the reversion prism, totally reflected from the longer base surface of the reversion prism, and then refracted again upon exiting from the other angled end surface of the reversion prism. If aligned properly, the optical signals exiting the reversion prism are received by a respective optical fiber of the second bundle. In order to facilitate this alignment and optical coupling of respective fibers of the first and second bundles, the reversion prism may be mounted upon a stage that permits the reversion prism to be controllably positioned. See, for example, U.S. Pat. No. 6,301,405.
The first and second bundles of optical fibers generally terminate with a respective optical collimation array. A conventional optical collimation array includes an outer sleeve or bearing that defines a lengthwise extending passage in which the end portions of the optical fibers are disposed. A collimating lens, such as a ball lens, may be associated with each optical fiber such that the signals emitted by the respective optical fiber are collimated by the ball lens before being launched through the internal cavity. At least one, if not both, of the optical collimation arrays is adapted to rotate about a longitudinal axis defined by the outer sleeve relative to the housing.
By appropriately aligning the optical collimation arrays and the reversion prism, however, the fiber optic rotary joint may maintain alignment of respective optical fibers of the first and second bundles as at least one of the first and second bundles rotates. In this regard, a fiber optic rotary joint is designed to rotate the housing at a rate that is 50% of the rate at which the bundle of optical fibers rotates. By rotating the housing at this rate, the reversion prism maintains optical alignment between respective optical fibers of the first and second bundles.
Fiber optic rotary joints are commonly utilized in sub-sea applications so as to couple first and second bundles of optical fibers in a manner that permits at least one of the bundles to rotate relative to the other. For example, fiber optic rotary joints may be utilized to optically couple the bundles of optical fibers that are utilized to communicate with underwater vehicles that may go to depths of 5,000 meters. In order to permit the fiber optic rotary joint to withstand the substantial compressive pressure experienced in a number of sub-sea applications in which the fiber optic rotary joint is at a substantial depth below sea level, the internal cavity defined by the housing of the fiber optic rotary joint is filled with an inert fluid, such as a halogenated hydrocarbon oil. Unfortunately, the optical properties of the fluid that fills the internal cavity of a fiber optic rotary joint vary significantly with changes in temperature and/or pressure due to corresponding changes in the density of the fluid. In particular, the index of refraction of the fluid varies as the temperature and/or pressure changes. As will be recognized, variations in the index of refraction of the fluid filling the internal cavity can significantly alter the angle at which the optical signals refract upon entering and exiting the reversion prism. Additionally, variations in the index of refraction of the fluid that fills the internal cavity of a fiber optic rotary joint alters the effective focal distance of the collimation lens. As such, the optical alignment between the first and second bundles of optical fibers is therefore diminished as the index of refraction of the fluid filling the internal cavity varies with changes in temperature and/or pressure, such as those experienced in a sub-sea environment. Accordingly, optical signals will not be coupled as efficiently, if at all, between the first and second bundles of optical fibers, thereby adversely affecting the optical performance of the fiber optic rotary joint.
As such, it would be desirable to provide an improved fiber optic rotary joint that further facilitates the alignment of first and second bundles of optical fibers, at least one of which is adapted to rotate relative to the other. In particular, it would be desirable to provide an improved fiber optic rotary joint that is capable of operating in a sub-sea environment in which the internal cavity has been filled with a fluid whose optical properties, including its index of refraction, may vary with changes in temperature and/or pressure. In particular, it would be desirable to provide a fiber optic rotary joint that maintains the alignment of the first and second bundles of optical fibers even as the optical properties of the fluid that fills the internal cavity vary.
In addition to the difficulties imparted by changes in the optical properties of the fluid, the optical fibers and the various optical components must also be properly aligned initially in order to avoid other sources of optical misalignment. In this regard, even a slight misalignment of the optical fibers will increase the insertion loss and, in some instances, render the fiber optic rotary joint unacceptable for at least some applications.
In order to properly align the first and second bundles of optical fibers, it is desirable that the optical fibers extend lengthwise through the respective outer sleeve with the longitudinal axes of the optical fibers being parallel to the longitudinal axis defined by the outer sleeve. As such, the outer sleeve is desirably sized such that the optical fibers are snugly received and held therewithin. This snug receipt of the optical fibers requires that the outer sleeve define an inner diameter very precisely with little tolerance. Such outer sleeves or bearings are therefore rarely, if ever, an off-the-shelf part and must be specifically manufactured to exacting tolerances, thereby increasing the overall cost of the fiber optic rotary joint. Even in instances in which the optical fibers are snugly received within the outer sleeve, the optical fibers may not necessarily be parallel to the longitudinal axis of the outer sleeve. In a common embodiment in which the bundle of optical fibers includes seven optical fibers arranged so as to have a central optical fiber and six optical fibers spaced thereabout, the six peripheral optical fibers may be twisted in a common direction, such as in a relatively helical pattern, about the central optical fiber. As such, the optical signals emitted by the bundle of optical fibers that are somewhat twisted may not be properly aligned with respective optical fibers of the other bundle of optical fibers, thereby reducing the efficiency with which optical signals are transmitted therebetween. Accordingly, each optical fiber and its respective collimation lens must generally be individually positioned so as to appropriately align the optic fiber and the associated collimating lens with the longitudinal axis defined by the sleeve. As will be recognized, this individual alignment can be quite time consuming. Once the optical fibers have been aligned, the optical fibers may be held in position by an epoxy. Unfortunately, the epoxy may cause the optical fibers to become slightly misaligned as the epoxy cures, thereby again disadvantageously increasing the insertion loss.